Logitech Marble Mouse drifting

My Logitech Marble mouse (http://www.logitech.com/index.cfm/products/details/CA/EN,CRID=2150,CONTENTID=5003) cursor is constantly drifting upward accross the screen -any idea why that would be?
It's a trackball mouse -if that makes any difference, and I *have* cleaned it out.
-But still: the cursor drifts slowly up the screen if left un-attended.
Any idea why that is -or, especially, how to fix it?
Thanks for your time.
(I know that a Mouse is a piece of Hardware -I just assumed that it is a driver/software issue that is causing the problem, so I decided to post to the Software forum)

 

I have an idea of what it may be. That is the only mouse I have used for the past 3 or 4 years. I am brutal with mine. I like to drag the mouse around.
Sometimes it's in my lap, or on the arm of my chair. I move it to
wherever I want my arm sitting at. Which means dragging the wire
all over the place. But I love that I can do that. Balltracks are the best!
But anyway, I have messed up the chord on 2 of them (from yanking
on the chord so often) and had to
replace them. And the first sign that I have damaged another chord
is when the mouse starts floating.

I finally put together a computer with front panel usb, and now I don't have to pull so hard on the cable to get the mouse where I want it.
Hence, my present mouse has lasted over a year without any new damage.